1. Field of the Invention
The present invention relates to a process of producing a polyurethane foam having flame resistance and more particularly, it relates to a process for producing a polyurethane foam having a high flame resistance and unaccompanied with a reduction in the properties of the polyurethane foam and with the formation of scorching. The invention further relates to the flame resistant polyurethane produced by such a process. In addition, the term "polyurethane foam" as used herein is intended to designate a "soft polyurethane foam."
2. Description of the Prior Art
Polyurethane foams have hitherto been used in a large number of fields due to their specific properties but have a serious disadvantage in that polyurethane foams are easily flamable.
Recently, however, regulations on the extent for rendering polyurethane materials flame retardant or flame resistant tend to become severer and severer, and the tendency is particularly remarkable in the application thereof in the fields of automobiles, railway vehicles, aircraft, etc.
The rendering of flame retarding properties or flame resistance to polyurethane foams is an important problem as described above but it has not yet been solved by conventional techniques. The methods which have been industrially employed at present still have a large number of difficulties.
Some techniques for rendering polyurethane foams flame retardant are described in the specifications of, e.g., Japanese Patent Publication Nos. 1750/'63, 9197/'70, 349/'64, 4846/'64, 8696/'64, 26335/'71, 21358/'69, 13037/'66, 2269/'71, etc., but these techniques are insufficient for satisfying the necessary requirements.
For example, Japanese Patent Publication Nos. 1750/'63, 9197/'70, etc. disclose the use of phosphorus-containing compounds or halogen-containing compounds as flame retarders for polyurethane foams but, in such known techniques, the phosphorus-containing compound must be added to such an extent that the content of phosphorus in the foam is higher than 1% or the flame retarder must be blended so that the content thereof is 5 to 30% by weight of the polyurethane foam. As a result of using such a large amount of flame retarder, the properties of the polyurethane foam are degraded, for example, the hardness is reduced, permanent set is increased, the strength is reduced, etc. Furthermore, when the polyurethane material is foamed excessively, the so-called scorching phenomenon occurs lowering the commercial value of the article. Moreover, since in such case the concentration of the flame retarder is high, the flame retarder tends to volatilize away due to the action of heat or with the passage of time, which results in a reduction in the flame proofing or flame resisting effects.
Also, in the polyurethane foam industry, various efforts have been to reduce the cost of polyurethane foams by increasing the effective utilization of the foams. For example, in case of making a cubic article of polyurethane foam from a crude foam with a curved surface layer, an attempt to increase the effective utilization of the crude foam is made by increasing the amount of the foam molded in one operation to reduce the amount of dust formed on cutting the article to as small as possible.
However, when a large amount of a phosphorus-containing compound or a halogen-containing compound is added to a foaming mixture as a flame retarder and the mixture is foamed into a large foam material, scorching occurs and in order to prevent the formation of such scorching, the content of the flame retarder must be reduced to as small as possible. Therefore, the developmment of a foaming technique which can render polyurethane foams flame retardant using as little an amount of a flame retarder as is possible or without using any flame retarder has been highly desired.
As described above, an important problem has been to improve the flame resistance of the polyurethane foam by blending a halogen-containing or phosphorus-containing compound in an amount as small as possible.
The extent of the reduction of the amount of retarder depends upon the extent of flame resistance required in the polyurethane foam and the properties of the flame retarder used but it is preferred that the amount be as small as possible and further it is most preferred that a flame retarder not be used at all.